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      ES6笔记
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    <h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>记的比较随便，格式可能不太好看，有需要补充的地方请私聊我~</p>
<h2 id="var"><a href="#var" class="headerlink" title="var"></a>var</h2><ol>
<li><p>重复声明</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">var</span> a<span class="token operator">=</span><span class="token number">12</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> a<span class="token operator">=</span><span class="token number">5</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>
<p>上边不会报错，最后a=5，在大型项目中如果重复声明一个变量直接崩盘。</p>
</li>
<li><p>控制修改</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">var</span> <span class="token constant">GIT_HOST</span><span class="token operator">=</span><span class="token string">'https://github.com/YuelinWang'</span><span class="token punctuation">;</span>
<span class="token keyword">if</span><span class="token punctuation">(</span><span class="token constant">GIT_HOST</span><span class="token operator">=</span><span class="token string">'error'</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<p>上边会直接把内容修改掉并进入到if中，敲的时候就要特别小心</p>
</li>
<li><p>块级作用域<br>let     防止重复声明，变量<br>const   防止重复声明，常量</p>
</li>
</ol>
<p>for循环中如果用var，想要console每一个i只简单的写是实现不了的- -，所以用闭包来实现。</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">let</span> bt<span class="token operator">=</span>document<span class="token punctuation">.</span><span class="token function">gerElementsByTagName</span><span class="token punctuation">(</span><span class="token string">'input'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span>bt<span class="token punctuation">.</span>length<span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">i</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        bt<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function-variable function">onclick</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
            <span class="token function">alert</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>我是真的烦闭包，一直弄不太清楚，但是如果用let，可以直接写，因为块级作用域</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">let</span> bt<span class="token operator">=</span>document<span class="token punctuation">.</span><span class="token function">gerElementsByTagName</span><span class="token punctuation">(</span><span class="token string">'input'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span>bt<span class="token punctuation">.</span>length<span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    bt<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function-variable function">onclick</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token function">alert</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>ES5的var的作用域——函数级<br>ES6的let的作用域——块级</p>
<p>块级即一个代码块——{}</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript">eg<span class="token operator">:</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span>
<span class="token keyword">if</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span>
<span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>以上三种均是代码块</p>
<p><strong>小结：</strong><br>var     重复声明，不能限制修改，函数级<br>let     不能，变量，块级<br>const   不能，常量，块级</p>
<h2 id="解构赋值"><a href="#解构赋值" class="headerlink" title="解构赋值"></a>解构赋值</h2><p>正常：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">let</span> a<span class="token operator">=</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> b<span class="token operator">=</span><span class="token number">2</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>

<p>解构赋值：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">let</span> json<span class="token operator">=</span><span class="token punctuation">&#123;</span>a<span class="token operator">:</span><span class="token number">12</span><span class="token punctuation">,</span>b<span class="token operator">:</span><span class="token number">5</span><span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">&#123;</span>a<span class="token punctuation">,</span>b<span class="token punctuation">&#125;</span><span class="token operator">=</span>json<span class="token punctuation">;</span>

<span class="token keyword">let</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">8</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>a<span class="token punctuation">,</span>b<span class="token punctuation">,</span>c<span class="token punctuation">]</span><span class="token operator">=</span>arr<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>注意：</p>
<ol>
<li>两边结构一样</li>
<li>右边必须是个东西</li>
<li>赋值和解构必须同时完成</li>
</ol>
<h2 id="箭头函数"><a href="#箭头函数" class="headerlink" title="箭头函数"></a>箭头函数</h2><pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span>
<span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">=></span><span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>举个栗子</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//排序</span>
<span class="token comment">//原数组</span>
<span class="token keyword">let</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">9</span><span class="token punctuation">,</span><span class="token number">22</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">//原来</span>
arr<span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">n1<span class="token punctuation">,</span>n2</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">return</span> n1<span class="token operator">-</span>n2<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//now</span>
arr<span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">n1<span class="token punctuation">,</span>n2</span><span class="token punctuation">)</span><span class="token operator">=></span><span class="token punctuation">&#123;</span><span class="token keyword">return</span> n1<span class="token operator">-</span>n2<span class="token punctuation">;</span><span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p><strong>简写</strong><br>注意事项：</p>
<ol>
<li>如果有且仅有一个参数，()也可以不写。</li>
<li>如果有且仅有一个语句，花括号也可以不写，注意必须是return，还是上边的例子。<br>举个栗子<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//排序</span>
<span class="token comment">//原数组</span>
<span class="token keyword">let</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">9</span><span class="token punctuation">,</span><span class="token number">22</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">//原来</span>
arr<span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">n1<span class="token punctuation">,</span>n2</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">return</span> n1<span class="token operator">-</span>n2<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//now</span>
arr<span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">n1<span class="token punctuation">,</span>n2</span><span class="token punctuation">)</span><span class="token operator">=></span><span class="token punctuation">&#123;</span><span class="token keyword">return</span> n1<span class="token operator">-</span>n2<span class="token punctuation">;</span><span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//简写</span>
arr<span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">n1<span class="token punctuation">,</span>n2</span><span class="token punctuation">)</span><span class="token operator">=></span>n1<span class="token operator">-</span>n2<span class="token punctuation">;</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre></li>
</ol>
<p><strong>修正this</strong></p>
<h2 id="…"><a href="#…" class="headerlink" title="…"></a>…</h2><p>参数/数组/</p>
<ol>
<li>收集</li>
<li>展开<br>举个栗子：<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//收集</span>
<span class="token keyword">function</span> <span class="token function">show</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span>b<span class="token punctuation">,</span><span class="token operator">...</span>c</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span>b<span class="token punctuation">,</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span>
<span class="token function">show</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">6</span><span class="token punctuation">,</span><span class="token number">10</span><span class="token punctuation">,</span><span class="token number">7</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//1进a，2进b，剩下全进c，成一个数组</span>

<span class="token comment">//展开</span>
<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">8</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">show</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span>b<span class="token punctuation">,</span>c</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token function">alert</span><span class="token punctuation">(</span>a<span class="token operator">+</span>b<span class="token operator">+</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span>
<span class="token function">show</span><span class="token punctuation">(</span><span class="token operator">...</span>arr<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//25</span>

<span class="token comment">//合并</span>
<span class="token keyword">let</span> arr1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> arr2 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>arr<span class="token punctuation">,</span><span class="token operator">...</span>arr1<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token function">alert</span><span class="token punctuation">(</span>arr2<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//json</span>
<span class="token keyword">let</span> json1<span class="token operator">=</span><span class="token punctuation">&#123;</span>a<span class="token operator">:</span><span class="token number">12</span><span class="token punctuation">,</span>b<span class="token operator">:</span><span class="token number">5</span><span class="token punctuation">,</span>c<span class="token operator">:</span><span class="token number">9</span><span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> json2<span class="token operator">=</span><span class="token punctuation">&#123;</span>
    <span class="token operator">...</span>json1<span class="token punctuation">,</span>
    d<span class="token operator">:</span><span class="token number">12</span>
<span class="token punctuation">&#125;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>json2<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre></li>
</ol>
<h2 id="map"><a href="#map" class="headerlink" title="map"></a>map</h2><p>映射，一一对应</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">let</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token number">68</span><span class="token punctuation">,</span><span class="token number">53</span><span class="token punctuation">,</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">98</span><span class="token punctuation">,</span><span class="token number">64</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token comment">//第一种</span>
<span class="token keyword">let</span> arr2<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>item<span class="token operator">>=</span><span class="token number">60</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token string">'及格'</span><span class="token punctuation">;</span>
    <span class="token keyword">else</span> <span class="token keyword">return</span> <span class="token string">'不及格'</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> item<span class="token operator">>=</span><span class="token number">60</span><span class="token operator">?</span><span class="token string">'及格'</span><span class="token operator">?</span><span class="token string">'不及格'</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//第二种</span>
<span class="token keyword">let</span> arr2<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">return</span> item<span class="token operator">>=</span><span class="token number">60</span><span class="token operator">?</span><span class="token string">'及格'</span><span class="token operator">?</span><span class="token string">'不及格'</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//第三种</span>
<span class="token keyword">let</span> arr2<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token operator">=></span>item<span class="token operator">>=</span><span class="token number">60</span><span class="token operator">?</span><span class="token string">'及格'</span><span class="token operator">?</span><span class="token string">'不及格'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>


console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>arr2<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="reduce"><a href="#reduce" class="headerlink" title="reduce"></a>reduce</h2><p>n个进入，出来1个</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//求平均数</span>
<span class="token keyword">let</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token number">68</span><span class="token punctuation">,</span><span class="token number">53</span><span class="token punctuation">,</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">98</span><span class="token punctuation">,</span><span class="token number">64</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">//tmp临时</span>
<span class="token comment">//当前数</span>
<span class="token comment">//当前位置</span>
<span class="token comment">//第一种</span>
<span class="token keyword">let</span> re<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">tmp<span class="token punctuation">,</span>item<span class="token punctuation">,</span>index</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">return</span> tmp<span class="token operator">+</span>item<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">alert</span><span class="token punctuation">(</span>re<span class="token operator">/</span>arr<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//第二种</span>
<span class="token keyword">let</span> re<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">tmp<span class="token punctuation">,</span>item<span class="token punctuation">,</span>index</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>index<span class="token operator">===</span>arr<span class="token punctuation">.</span>length<span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token punctuation">(</span>tmp<span class="token operator">+</span>item<span class="token punctuation">)</span><span class="token operator">/</span>arr<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
    <span class="token keyword">else</span> <span class="token keyword">return</span> tmp<span class="token operator">+</span>item<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">alert</span><span class="token punctuation">(</span>re<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="filter"><a href="#filter" class="headerlink" title="filter"></a>filter</h2><p>过滤</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//求平均数</span>
<span class="token keyword">let</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token number">68</span><span class="token punctuation">,</span><span class="token number">53</span><span class="token punctuation">,</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">98</span><span class="token punctuation">,</span><span class="token number">64</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token comment">//1</span>
<span class="token keyword">let</span> arr2<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token operator">=></span><span class="token punctuation">&#123;</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>item<span class="token operator">%</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    <span class="token keyword">else</span> <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//2</span>
<span class="token keyword">let</span> arr2<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token operator">=></span><span class="token punctuation">&#123;</span>
    <span class="token keyword">return</span> item<span class="token operator">%</span><span class="token number">2</span><span class="token operator">===</span><span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//3</span>
<span class="token keyword">let</span> arr2<span class="token operator">=</span>arr<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token operator">=></span>item<span class="token operator">%</span><span class="token number">2</span><span class="token operator">===</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>arr2<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="forEach"><a href="#forEach" class="headerlink" title="forEach"></a>forEach</h2><p>没有返回值，遍历</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">let</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token number">68</span><span class="token punctuation">,</span><span class="token number">53</span><span class="token punctuation">,</span><span class="token number">12</span><span class="token punctuation">,</span><span class="token number">98</span><span class="token punctuation">,</span><span class="token number">64</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

arr<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">item<span class="token punctuation">,</span>index</span><span class="token punctuation">)</span><span class="token operator">=></span><span class="token punctuation">&#123;</span>
    <span class="token comment">//1</span>
    <span class="token function">alert</span><span class="token punctuation">(</span><span class="token string">'第'</span><span class="token operator">+</span>index<span class="token operator">+</span><span class="token string">'个：'</span><span class="token operator">+</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//2     反单引号    左上角1左边的</span>
    <span class="token function">alert</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">第</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">$&#123;</span>index<span class="token interpolation-punctuation punctuation">&#125;</span></span><span class="token string">个：</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">$&#123;</span>item<span class="token interpolation-punctuation punctuation">&#125;</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="json"><a href="#json" class="headerlink" title="json"></a>json</h2><pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//注意都要有双引号</span>
<span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">&#123;</span>a<span class="token operator">:</span><span class="token number">12</span><span class="token punctuation">,</span>b<span class="token operator">:</span><span class="token number">5</span><span class="token punctuation">&#125;</span></span><span class="token punctuation">)</span>      <span class="token operator">=></span>  <span class="token string">'&#123;"a":12,"b":5&#125;'</span>
<span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'&#123;"a":12,"b":5&#125;'</span><span class="token punctuation">)</span>    <span class="token operator">=></span>  <span class="token punctuation">&#123;</span>a<span class="token operator">:</span><span class="token number">12</span><span class="token punctuation">,</span>b<span class="token operator">:</span><span class="token number">5</span><span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<h2 id="参考链接"><a href="#参考链接" class="headerlink" title="参考链接"></a>参考链接</h2><p><a target="_blank" rel="noopener" href="https://www.bilibili.com/video/BV1wt411t7hg">【智能社】ES6精讲—主讲老师：石川（Blue）—高清版本</a></p>

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